Among "neriseihin" products prepared from fish or shellfish paste meat such as surimi or minced meat of fish and shellfish are known as Japanese food kamaboko, chikuwa and fish meat sausage. These "neriseihin" or fish or shellfish paste meat products are extensively marketed as food. To produce such "neriseihin", the edible part of fish and shellfish is kneaded with salt, seasoning, suitable additives and flavoring agents to produce paste meat, i.e., surimi and this paste meat is subjected to molding and heating and setting processes, in sequence, thus forming a fish or shellfish paste meat product or neriseihin having an elastic structure. A feature of the "neriseihin" is that the setting of the paste meat produces a three-dimensional net-work structure of protein or the like, thus providing a certain elasticity. Recently, a tendency toward the food taste is changing with westernization and Americanization and diversification of the living standards, and accordingly kamaboko, chikuwa, fish and shellfish sausage and other fish and shellfish "neriseihin" are improved to meet western and American requirements. However, these products are not essentially improved. More specifically, even in improved neriseihin products the net-work structure of protein remains as such and cannot always satisfy demands for westernization and Americanization and diversification of the diet. Further, for an enhancement of health it is demanded that the salt content be as low as possible or zero. Nevertheless, for producing surimi from the edible part of fish and shellfish it is necessary to add a comparatively large amount of salt so as to provide a certain degree of viscosity, by melting a protein contained in the edible part by addition of salt. This salt addition is undesired, however, from the health standpoint.
As an example, Japanese Patent Publication 64-27 and Japanese Patent Disclosure 62-259565 disclose using surimi of fish and shellfish as a raw material, subjecting it to cutting and kneading with salt or other additives and flavoring agent and seasoning agents to obtain surimi, producing strip-like moldings of the surimi, heating and setting the moldings to obtain a neriseihin product, i.e., kamaboko, then cutting the product lengthwise with a knife or the like to obtain many filaments and, then integrating these filaments by adding a binder of surimi to them and heating the integrated filaments using steam to produce neriseihin products resembling scallop meat or king crab leg meat. These food products are obtained by mechanically cutting "neriseihin" having usual kamaboko structure to form a fibrous structure like that of crab leg meat. Therefore, their appearance closely resembles natural scallop meat or king crab leg meat so that they can hardly be discriminated. However, although the individual filaments are thin and elongate, they retain a so-called kamaboko structure, i.e., a net-work structure, in which protein or the like is three-dimensionally coupled in a net like form. That is, their structure is essentially different from the structure of natural scallop meat or king crab meat. In addition, the conventional "neriseihin" product having a kamaboko structure contains a comparatively large amount of salt, which is undesired form the health standpoint.
Japanese Patent Disclosure 63-68059 and Japanese Patent Disclosure 63-68060 disclose methods of producing a fibrous neriseihin product consisting of a bundle of aligned fibrous elements, containing protein or the like linearly orientated, in order to closely resemble the structure of crab leg or shrimp with the structure of the neriseihin itself. These methods are developed by utilizing a conventional technique in that a twin screw extruder is used for a kneading step to prepare a bread paste raw material. In detail, fish or shellfish material such as surimi is mixed with additives and seasoning agents to obtain a sol material, which is then converted into a gel material by heating it or adjusting to high viscosity. The thus-obtained gel material having high viscosity is then kneaded and thermally fused in a twin-screw extruder. The fused material is then extruded via an outlet of the leading end of the extruder, thereby carrying out an aligned orientation of protein molecules or the like by making use of a shearing effect of the twin-screw extruder. Subsequently, each of the resultant aligned filaments is separated by using water. Then, the separated aligned filaments are mixed with fish and shellfish surimi containing salt, and the resultant mixture is molded into a desired shape and then set, followed by boiling or heating, resulting in a fibrous neriseihin product. The neriseihin which is obtained in this way, unlike the prior art kamaboko structure, has a structure, in which filaments having protein molecules aligned in a direction are randomly coupled together and integrated. This structure resembles the structure of natural crab or shrimp meat and fits the aim of westernization, Americanization, or diversification of living standards to a certain extent. These methods utilizes the conventional technique in that a twin-screw extruder is used in a kneading process to prepare bread paste raw material having high viscosity. In other words, each of these methods is merely a method in which, instead of bread paste raw material, fish and shellfish raw material is processed with a twin-screw extruder to carry out an orientation of filaments consisting of protein linearly aligned. However, owing to unsatisfactory orientation of filaments toward one direction, these oriented filaments are respectively separated and then randomly coupled together. For easily processing fish and shellfish material with a twin-screw extruder in this method, the sol material having low viscosity is pre-heated or adjusted to high viscosity by adding starch, salt, etc. in order to change its character to be close to the character of bread paste raw material. However, in spite of the fact that a product free from salt is desired, without addition of salt a viscosity like that of bread paste raw material cannot be obtained. Therefore, a desired product free from salt cannot be produced. In addition, a preparation of raw material having high viscosity owing to a twin screw extruder having a prior art structure cannot give a rise to satisfactory orientation of protein or the like, resulting in a product having imperfect protein orientation structure, which cannot be used as a desired marketable product. Accordingly, the unsatisfactory orientated product processed through a twin-screw extruder is disjointed by agitating it in water. The disjointed elements are then mixed with salt-containing surimi as binder, and the mixture is steamed or boiled to obtain a neriseihin as a marketable product. In such a method, a finishing process is required after the process in the twin-screw extruder, and therefore, the overall process of manufacture is complicated. In addition, in the finishing process, the product having a slightly imperfectly oriented structure is disjointed, and the disjointed elements are mixed with a binder before molding into a desired form. Therefore, it is impossible to obtain a product, in which filaments having a satisfactory orientation of protein molecules, are uniformly aligned in a certain direction.
In greater detail, a prior art extruder, which is used to prepare a bread paste raw material or like, comprises mainly a cylindrical barrel and a screw rotatable therein. The screw is provided on a screw shaft. A raw material mainly composed of flour is transported by the action of the screw, and during this time individual components of the material are kneaded together to obtain bread paste raw material. The raw material in the cylindrical barrel can be heated. The heating is usually effected by heating the cylindrical barrel itself with steam or an electric heater or by introducing steam into the material. The outlet end of the cylindrical barrel is throttled to form a nozzle or an orifice. With this throttled end, the material being transported by the screw is pressurized to increase its pressure. As an extruder, a uni-screw extruder with a single screw rotatably accommodated in a cylindrical barrel has been used, but recently there is a trend for using a twin-screw extruder, which has high material transportation capacity compared to the uni-screw extruder. The twin-screw extruder is capable of having a change of screws depending on the characteristics of the material and purpose of processing, and also it can process materials having high oil contents. For these reasons, it is used for preparing bread paste material.
FIG. 1 shows part of a typical example of the same direction rotation type twin-screw extruder. As it is shown, two screw shafts 2 and 3 are provided for rotation in the same directions in cylindrical barrel 1. Screws 21 and 31 on these screws shafts 2 and 3 are rotated in mesh with each other. Flock nuts 22 and 32 are provided at the ends of screw shafts 2 and 3, and barrel die 4 is provided in the vicinity of flock nuts 22 and 32. Barrel die 4 has a substantially central orifice or nozzle-like outlet 41. In cylindrical barrel 1, two screw shafts 2 and 3 are rotated in the same directions. With this rotation, material is kneaded and compressed while it is transferred by screws 21 and 31. During this time, material is heated by a heating element (not shown) in cylindrical barrel 1 and internal heat is generated at the time of applying shearing stress to the material by screws 21 and 23. Since the end of cylindrical barrel 1 is throttled with barrel die 4, material is pressurized as it is transferred. With a twin-screw extruder this pressure is increased to a pressure higher than the steam pressure. Therefore, neither effervescence nor splash takes place in cylindrical barrel 1. As material is extruded from outlet 41 of barrel die 4 into atmosphere, it is inflated to generate high pressure steam, so that the material is liable to be scattered and become fine particles. That is, a phenomenon of flashing is liable to occur to spoil the moldability of the product. Particularly, when the material has high viscosity and contains little moisture, this trend is promoted to spoil regular orientation, and a spructure having uniformly aligned filaments cannot be obtained.
To avoid this phenomenon, it is in practice to cool the material at outlet 41 of barrel die 4. However, even by cooling the material in this way the flashing cannot be perfectly avoided. Rather, when the material is cooled locally at outlet 41, clogging is caused to vary the pressure in cylindrical barred 1 and sometimes result in explosive discharge of material. Therefore, it is impossible to control the discharge and to obtain a molding having a desired shape.
U.S. Pat. No. 4,816,278 discloses a method, in which, for avoiding the flashing when processing the fish or shell fish raw material, a long tubular nozzle is coupled to the end of a cylindrical barrel such as to hold the material in a fused state in a first half of the nozzle while cooling the material to 100.degree. C. or below in the second half of the nozzle, thus attaining a fibrous structure. However, while the flashing can be avoided with this long nozzle, the cooling zone is constituted by only one half of the length of the nozzle and is insufficient for providing an aligned orientation. Besides, the nozzle having the first heating or fusing zone and second cooling zone has a considerable overall length and offers high contact resistance. Therefore, material cannot be continuously and steadily supplied into the long tubular nozzle only with the sole extruding force of the twin-screw extruder.